It is well known in the audio/video arts that AVS (Audio Video and Subcode) data may be recorded onto magnetic tape for reproduction at a later time. A typical recording/reproducing apparatus implements a helical scanning method in which the AVS data is recorded in tracks that are obliquely formed on the magnetic tape by rotary heads. FIG. 1 illustrates the formation of helical tracks on a magnetic tape. As seen in the figure, the tracks are formed so that each track overlaps with its neighboring tracks. In an NTSC compatible recording, a video frame is recorded in ten tracks. In a PAL compatible recording, a video frame is recorded in twelve tracks. The recording shown in FIG. 1 is a NTSC compatible recording, with shading provided to identify individual frames. The tracks within each frame are numbered sequentially from 0 to 9.
FIG. 2 shows the format of an individual track. The left side of the track is the head entrance side and the right side is the head exit side. The track includes an ITI (Insert and Track Information) area, an audio data area, a video data area, and a subcode area. Interspersed between these areas are IBGs (Inter Block Gaps), in which no data is recorded. The audio data, video data, and subcode data are collectively referred to as AVS data. The ITI area contains administrative information about the track, for example information about the track's size, or information about the frame in which the track belongs.
There are many applications for a system of the foregoing type. One such application is the editing of a previously recorded tape. Often, when editing a previously recorded tape, it is desirable to overwrite the AVS portion of previously recorded tracks without disturbing the ITI portion of those tracks. Track overwriting of this kind is hereinafter referred to as "AVS insertion".
In order to successfully perform an AVS insertion, it is necessary to match the phase of the frame for the original AVS data (hereinafter "the background data") to the phase of the frame for the new AVS data. The phase of the frame for the original AVS data may be obtained from the data itself, however this presents a problem since the original data is overwritten during the insertion. Often, this problem is addressed by including one or more pre-reading heads in the system for the purpose of reading the original track(s) and determining the phase of the background frame prior to overwriting. The problem with the pre-reading method is that it cannot correct for deviations of frame phase that might occur during the insertion.
FIGS. 3A and 3B illustrate one type of frame phase deviation that may occur during an AVS insertion performed according to the pre-reading method. FIG. 3A shows an original NTSC recording that contains a discontinuity at the point indicated by the asterisk. As can be seen from the figure, the original recording includes an incomplete frame that has only 6 tracks instead of the usual 10. When the pre-reading method is used to write over the recording of FIG. 3A, the recording of FIG. 3B results. The deviation apparent in FIG. 3B is caused by the inability of the pre-reading heads to recognize the discontinuity in the original recording. As the insertion is performed the pre-reading heads reach the 0th track of the incomplete original frame and determine the frame phase from the information in that track. The tracks of the new frame are then sequentially written over the tracks of the original frame without regard to the discontinuity. Thereby resulting in an incorrect matching of the new AVS data and old ITI data.
FIGS. 4A and 4B illustrate a second type of frame phase deviation that may occur during an AVS insertion performed according to the pre-reading method. Unlike the tracks in FIG. 3A, the tracks in FIG. 4A are correctly formed in accordance with the NTSC system. In this second illustration the frame phase deviation is caused when the system's servo is disturbed during the AVS insertion. For example, the servo may experience a mechanical shock during insertion. As can be seen from FIG. 4B, such a disturbance results in incorrect placement of the newly written tracks, and therefore gives rise to a frame phase deviation.